Air-fuel ratio control apparatus for engine

ABSTRACT

An engine has an upstream catalyst and a downstream catalyst in an exhaust system. An air-fuel ratio control apparatus has an oxygen sensor that outputs signal indicative of an oxygen storage amount in the downstream catalyst. The apparatus has an air supply device for supplying the air into upstream the downstream catalyst. When the signal of the oxygen sensor indicates a shortage of the oxygen storage amount in the downstream catalyst, the air supply device is activated to supply the air to the downstream catalyst. Thus, the downstream catalyst can recover the oxygen storage amount sufficient to keep its catalysis.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is based on Japanese Patent Application No. 2002-33078 filed on Feb. 8, 2002, the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine.

[0004] 2. Description of Related Art

[0005] Conventionally, a catalyst such as a three-way catalyst is disposed in an exhaust path of an internal combustion engine for purifying exhaust gas. In such the systems, a fuel quantity supplied to the engine is controlled to maintain an air-fuel ratio within a specific range in which the catalyst efficiently works. For example, a gas sensor for detecting an oxygen concentration in the exhaust gas is used, and the fuel quantity is adjusted to keep an output signal of the gas sensor within a specific range. In this case, the output signal of the gas sensor indicates an air-fuel ratio of air-fuel mixture supplied to the engine.

[0006] In addition, two or more catalysts may be disposed in the exhaust path in a series manner. In this case, if a downstream one of the catalysts completely consumes an oxygen storage amount, the performance of the downstream catalyst is decreased and emission may become worth.

SUMMARY OF THE INVENTION

[0007] It is an object of the present invention to provide an air-fuel ratio control apparatus for an engine that is capable of preventing emission.

[0008] It is another object of the present invention to provide an air-fuel ratio control apparatus for and engine that is capable to preventing emission by keeping a performance of a downstream catalyst.

[0009] It is a still another object of the present invention to keep a performance of a downstream catalyst by supplying oxygen rich exhaust gas, i.e., fuel lean exhaust gas, to the downstream catalyst in order to recover an oxygen storage in the downstream catalyst.

[0010] According to a first aspect of the present invention, the air-fuel ratio control apparatus for an engine comprises a first catalyst disposed in an exhaust passage of the engine, and a second catalyst disposed in the exhaust passage downstream the first catalyst. Therefore, the second catalyst works as the downstream catalyst. The apparatus further comprises an air-fuel ratio sensor disposed in the exhaust passage upstream the first catalyst, second catalyst condition detecting means for detecting a condition of the second catalyst, and feedback control means for controlling an air-fuel ratio to maintain a detected air-fuel ratio by the air-fuel ratio sensor in a target air-fuel ratio by using a feedback control method, the target air fuel ratio being set in accordance with an output of the second catalyst condition detecting means. Therefore, the air-fuel ratio supplied to the engine is controlled to maintain the contents of the exhaust gas upstream the first catalyst in a target contents. The apparatus comprises an oxygen storage recovery means for recovering an oxygen storage in the second catalyst when the second catalyst condition detecting means outputs a predetermined level. Although, the second catalyst may consume the oxygen storage under the feedback control of the feedback control means, the oxygen storage recovery means allows to recover the oxygen storage in the second catalyst in response to the output of the second catalyst condition detecting means. As a result, even if the air-fuel ratio is mainly controlled in response to the air-fuel ratio sensor disposed upstream the first catalyst, it is possible to keep the performance of the second catalyst.

[0011] The second catalyst condition detecting means may detect an air-fuel ratio of the exhaust gas in the exhaust passage downstream the second catalyst, or a rich condition and a lean condition of the exhaust gas in the exhaust passage downstream the second catalyst.

[0012] The oxygen storage recovery means may increase an oxygen concentration in the exhaust gas supplied to the second catalyst when the second catalyst condition detecting means outputs a signal that indicates a rich condition of the exhaust gas. The rich condition is a fuel rich condition.

[0013] The apparatus may further comprise an air supply device that supplies the air to the second catalyst. In this case, the oxygen storage recovery means may activate the air supply device in order to increase the oxygen concentration.

[0014] The oxygen storage recovery means may at least reduce a fuel amount supplied to the engine in order to increase the oxygen concentration. The fuel amount may be reduced by adjusting the air-fuel ratio of the exhaust gas supplied to the second catalyst in a lean condition. The target air-fuel ratio may be set in a lean value in order to reduce the fuel supply. The apparatus may further comprise means for suspending a fuel supply for all or partial cylinder and resuming the fuel supply when an engine speed reaches to a resuming engine speed. In this case, the fuel amount may be reduced by adjusting the resuming engine speed lower than a normal value.

[0015] The feedback control means may comprise main feedback means for maintaining the detected air-fuel ratio in the target air-fuel ratio by using the feedback control method, and sub feedback means for setting the target air fuel ratio in accordance with the output of the second catalyst condition detecting means.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Features and advantages of embodiments will be appreciated, as well as methods of operation and the function of the related parts, from a study of the following detailed description, the appended claims, and the drawings, all of which form a part of this application. In the drawings:

[0017]FIG. 1 is a block diagram of an engine control system including an air-fuel ratio control apparatus according to a first embodiment of the present invention;

[0018]FIG. 2 is a flowchart showing an air supply control according to the first embodiment of the present invention;

[0019]FIG. 3 is a timing diagram showing waveforms of signals according to the first embodiment of the present invention;

[0020]FIG. 4 is a flowchart showing a fuel control according to a second embodiment of the present invention;

[0021]FIG. 5 is a flowchart showing details of the flowchart in FIG. 4;

[0022]FIG. 6 is a flowchart showing a target air-fuel ratio setting process according to the second embodiment of the present invention;

[0023]FIG. 7 is a timing diagram showing waveforms of signals according to the second embodiment of the present invention;

[0024]FIG. 8 is a flowchart showing a fuel cut control according to a third embodiment of the present invention;

[0025]FIG. 9 is a timing diagram showing waveforms of signals according to the third embodiment of the present invention;

[0026]FIG. 10 is a block diagram of an engine control system according to a fourth embodiment of the present invention;

[0027]FIG. 11 is a flowchart showing an air supply control according to the fourth embodiment of the present invention;

[0028]FIG. 12 is a timing diagram showing waveforms of signals according to the fourth embodiment of the present invention;

[0029]FIG. 13 is a flowchart showing a target air-fuel ratio setting process according to a fifth embodiment of the present invention;

[0030]FIG. 14 is a timing diagram showing waveforms of signals according to the fifth embodiment of the present invention;

[0031]FIG. 15 is a flowchart showing a fuel cut control according to a sixth embodiment of the present invention; and

[0032]FIG. 16 is a timing diagram showing waveforms of signals according to the sixth embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0033] First Embodiment

[0034] Referring to FIG. 1, an internal combustion engine (engine) is a spark ignition type four-stroke multi-cylinder engine for a motor vehicle. The engine 1 has an air cleaner 2 for filtering and cleaning an intake air. The air cleaner 2 is connected with an intake passage 3 in which the intake air flows. A throttle valve 4 for adjusting an amount of the intake air is disposed in the intake passage 3. The intake passage 3 has a surge tank 5. The intake passage 3 is connected with an intake manifold 6. The intake manifold 6 provides a plurality of branch passages leading to cylinders of the engine respectively. Each of the branch passage has an injector 7 for injecting fuel and mixing the injected fuel with the intake air to provide mixture. In this embodiment, the engine 1 has four cylinders. Each of the cylinders has a spark plug 8. The spark plug 8 is directly connected with an igniter device 9 that supplies high-tension voltage to the spark plug 8 in response to an ignition signal from an engine control unit.

[0035] The engine 1 has an exhaust manifold 11 and an exhaust passage 12. Two catalysts 13, 14 are disposed in the exhaust passage 12 in a series manner. The catalyst 13 is disposed in the exhaust passage 12 upstream the catalyst 14. The catalyst 14 is disposed in the exhaust passage 12 downstream the catalyst 13. The catalysts 13, 14 are three-way type that carries catalytic components such as Pt and Rh and additives such as Ce and La. The catalysts 13, 14 purify the exhaust gas and decrease emissions by decreasing emission components such as CO, HC and NOx. The catalyst 13 and 14 are distanced each other. The catalyst 13 may have a plurality of catalysts. The catalyst 14 may also have a plurality of catalysts. The catalyst 14 is located on a downstream side of at least one of the other catalyst.

[0036] An air supply port is formed on the exhaust passage 12 between the catalysts 13 and 14. An air supply device 15 is connected to the air supply port. The air supply device 15 has, for example, an air pump for supplying the air into the exhaust passage 12, a reed valve, and an electromagnetic ON/OFF valve. Therefore, the air supply device is controllable in a supplying condition and a stopping in response to a signal from an engine control unit.

[0037] An airflow meter 21 is disposed in the intake passage 3 just downstream the air cleaner 2. The airflow meter 21 detects an intake air amount GA that is an intake air amount per unit time. A throttle sensor 22 is disposed on the throttle valve 4, and outputs a signal indicative of an opening degree TA of the throttle valve 4 and a signal indicative of a fully closed state of the throttle valve 4. An idle switch may be disposed to detect the fully closed position. An intake pressure sensor 23 is disposed on the surge tank 5 and outputs a signal indicative of an intake pressure PM. A water temperature sensor 24 is disposed on a cylinder block of the engine 1 and outputs a signal indicative of a water temperature THW of cooling water. A crank angle sensor 25 is disposed on the engine 1 and outputs a signal indicative of an engine speed NE of rotation of the engine.

[0038] A first gas sensor 26 is disposed on the exhaust passage 12 just upstream the catalyst 13. The first gas sensor 26 is an air-fuel ratio sensor, i.e., an A/F sensor. The first gas sensor 26 outputs a signal VOX1 indicative of an air-fuel ratio based on the contents of the exhaust gas in the exhaust passage 12 just upstream the catalyst 13. A second gas sensor 27 is disposed on the exhaust passage 12 between the catalyst 13 and the catalyst 14. The second gas sensor 27 is an oxygen concentration sensor, i.e., an 02 sensor. The second gas sensor 27 outputs a signal VOX2 indicative of a rich condition (fuel rich condition) or a lean condition (fuel lean condition) of the exhaust gas in the exhaust passage 12 between the catalysts 13, 14. The lean or rich condition of the exhaust gas in the exhaust passage just downstream the catalyst 14 can be estimated based on the signal VOX2.

[0039] An engine control unit (ECU) 30 is mounted on the vehicle. The ECU 30 is constructed as a microcomputer having well known components such as CPU, ROM, RAM, Back-up RAM, and I/O. The ECU 30 is connected with the sensors to input an engine operating conditions such as the intake air amount GA, the throttle opening degree TA, the intake pressure PM, the water temperature THW, the engine speed NE, and the signals VOX1, VOX2. The ECU 30 is connected with actuators such as the injector 7 to control the engine operating conditions. The ECU 30 outputs control signals such as an injection amount signal TAU for the injector 7, an ignition timing signal Ig for the igniter 9, and an air supply signal DV for the air supply device 15.

[0040] The signal VOX1 from the first gas sensor 26 is mainly used to evaluate an air-fuel ratio of the mixture. The signal VOX1 from the second gas sensor 27 is used to detect a rich condition or a lean condition of the exhaust gas. The signal VOX1 is also used to detect a rich-lean change over timing from the rich condition to the lean condition and a lean-rich change over timing from the lean condition to the rich condition.

[0041] The ECU 30 executes an air-fuel ratio control. The ECU 30 provides a main feedback controller 30 a and a sub feedback controller 30 b. The main feedback controller 30 a controls at least the injector 7 to adjust the fuel amount based on the signal VOX1 and a target air-fuel ratio at a location upstream the catalyst 13. The main feedback controller 30 a controls the fuel amount so that the signal VOX1 indicates the target air-fuel ratio. The sub feedback controller 30 b controls the target air-fuel ratio in the main feedback controller 30 a in accordance with the signal VOX2. The ECU 30 further executes a storage amount control as shown in FIGS. 2 and 3. FIG. 2 is a flowchart showing processing of the ECU 30. FIG. 3 is a timing diagram showing conditions of the engine 1 under the control of the ECU 30. A routine shown in FIG. 2 is executed every predetermined interval.

[0042] Referring to FIG. 2, in S101, it is determined that whether or not a timer EXTA is equal to 0. The timer EXTA represents a period of time for driving the air supply device 15. In S102, it is determined that whether or not the signal VOX2 is greater than a threshold value, e.g., 0.55 (V). The threshold value 0.55 (V) is a threshold value between a rich condition (fuel rich condition) and a lean condition (fuel lean condition) of the exhaust gas at a location downstream the catalyst 14. Therefore, in S102, the condition of the exhaust gas downstream the catalyst 14 is estimated by the signal VOX2 indicative of the condition of the exhaust gas upstream the catalyst 14. If the signal VOX2 is higher than 0.55 (V), the exhaust gas is the rich condition.

[0043] If the signal VOX2 is gradually increased and reaches 0.55(V) at t00 as shown in FIG. 3, the routine proceeds to S103 to accumulate the intake air amount GA. In S103, the present value of the intake air amount GA is added to the accumulated amount GASUM. In S104, it is determined that whether or not the accumulated amount GASUM is greater than a threshold value GASUMMIN. If the accumulated amount GASUM is not greater than the threshold value GASUMMIN, the routine proceeds to S111 to deactivate the air supply device 15. The air supply device 15 is maintained turned off from t00 to t01 as shown in FIG. 3.

[0044] If the accumulated amount GASUM reaches to the threshold value GASUMMIN at t01 as shown in FIG. 3, the routine proceeds to S105 and S106 to initialize the accumulated amount GASUM and preset the timer EXTA. In S105, the accumulated value GASUM is initialized to 0. In S106, a predetermined value K is set in the timer EXTA. Then, in S107, it is determined whether or not the intake air amount GA is smaller than a threshold value GAMIN. If the intake air amount GA is maintained below the threshold GAMIN as shown in ranges from t01 to t02, and from t03 to t05 in FIG. 3, the routine proceeds to S108 and S109 to decrement the timer EXTA and activate the air supply device 15. In S108, the timer EXTA is decremented by a predetermined value, e.g., 1. Therefore, the timer EXTA is decreased from t01 to t02 and from t03 to t05 in FIG. 3. In S109, the air supply device 15 is activated to supply air to the catalyst 14. As a result, even if the catalyst 14 may consume all of the oxygen storage during the feedback control of the main and sub feedback controller 30 a, 30 b, it is possible to recover the oxygen storage in the catalyst 14 by supplying the air. The oxygen storage is recovered to a certain level, and the catalyst 14 performs catalysis efficiently.

[0045] If the intake air amount GA become not smaller than the threshold value GAMIN as shown in a period from t02 to t03 in FIG. 3, the routine proceeds from S107 to S111 to deactivate the air supply device 15. Therefore, the air supply to the catalyst 14 is stopped from t02 to t03 in FIG. 3.

[0046] If once the timer EXTA is preset in S106, the routine jumps from S101 to S107 until the timer EXTA become 0. Therefore, if once the timer EXTA is preset, the air supply device 15 works for a predetermined time corresponding to the predetermined value K as shown in a period from t01 to t05 in FIG. 3. If the timer EXTA is equal to 0 and VOX2 is not greater than 0.55(V), the routine proceeds to S110 and S111 to initialize the accumulated amount GASUM and deactivate the air supply device 15. Therefore, when the exhaust gas supplied to the catalyst 14 is maintained at the lean condition, the air supply device 15 is never activated.

[0047] According to the first embodiment, it is possible to prevent the catalyst 14 from shortage of the oxygen storage.

[0048] Second Embodiment

[0049] The second embodiment has the same components as the first embodiment. FIG. 4 is a flowchart showing a fuel amount control including an air-fuel ratio control corresponding to the main feedback controller 30 a and the sub feedback controller 30 b. The routine in FIG. 4 is executed every predetermined crank angle. In the second embodiment, when a shortage of the oxygen storage in the catalyst disposed downstream of another catalyst is estimated, the target air-fuel ratio is set in a leaner value to recover the oxygen storage in the catalyst.

[0050] In S201, a base fuel amount TP is computed based on an operating condition of the engine 1 such as the intake pressure PM, and the engine speed NE. In S202, it is determined that whether or not the closed loop air-fuel ratio control, i.e., feedback control is permitted. The permission is established when the water temperature THW si higher than a predetermined temperature, and the engine 1 is not operated under a high speed or heavy load condition.

[0051] If the closed loop control is permitted, in S203, the ECU 30 sets a target air-fuel ratio λtg. The target air-fuel ratio λtg indicates a target air-fuel ratio of the exhaust gas upstream the catalyst 13. In S204, the ECU 30 computes a coefficient FAF for correcting the fuel amount in accordance with a difference between the target air-fuel ratio λtg and an actual air-fuel ratio indicated by the signal VOX1 from the first gas sensor 26. On the other hand, if the closed loop control is not permitted, in S205, the coefficient FAF is set at 1.0.

[0052] In S206, the ECU 30 computes the injection amount signal TAU based on the basis fuel amount TP, the coefficient FAF and the other corrective coefficients FALL by TAU=TP×FAF×FALL.

[0053]FIG. 5 is a flowchart showing a target air-fuel ratio setting routine corresponding to the sub feedback controller 30 b. In S301, it is determined that whether or not the signal VOX2 is greater than a threshold value, e.g., 0.6 (V). The threshold value 0.6(V) represents a threshold between a rich condition and a lean condition of the exhaust gas between the catalyst 13 and the catalyst 14.

[0054] If the signal VOX2 is not greater than 0.6(V), it is assumed that the exhaust gas is the lean condition. In S302, it is determined that whether or not the last determination of S301 is the lean condition.

[0055] In a case that the last determination is also the lean condition, the routine proceeds to S303 and 304 to set a rich accumulated amount λIR and set the target air-fuel ratio λtg. In S303, the rich accumulated amount λIR is obtained by looking up a map defined by a present value of the intake air amount GA. The map is defined so that the rich accumulated amount λIR is decreased as the intake air amount GA is increased. In S304, the target air-fuel ratio λtg is corrected to a fuel rich side by the rich accumulated amount λIR.

[0056] In a case that the last determination is the rich condition, it is assumed that the exhaust gas condition has been changed from the rich to the lean condition. The routine proceeds to S305 and 306 to set a rich skip amount λSKR and set the target air-fuel ratio λtg. In S305, the rich skip amount λIR is obtained by looking up a map defined by an estimated storage amount of the catalyst 14. The estimated storage amount of the catalyst 14 is estimated based on the signal VOX2 from the second gas sensor 27. The map is defined so that the rich skip λSKR is increased as the storage amount of lean components in the catalyst 14 is increased. In S306, the target air-fuel ratio λtg is corrected to a fuel rich side by the sum of the rich accumulated amount λIR and the rich skip amount λSKR.

[0057] If the signal VOX2 is greater than 0.6(V), it is assumed that the exhaust gas is the rich condition. In S307, it is determined that whether or not the last determination of S301 is the rich condition.

[0058] In a case that the last determination is also the rich condition, the routine proceeds to S308 and 309 to set a lean accumulated amount λIL and set the target air-fuel ratio λtg. In S308, the lean accumulated amount λIL is obtained by looking up a map defined by a present value of the intake air amount GA. The map is defined so that the lean accumulated amount λIL is decreased as the intake air amount GA is increased. In S309, the target air-fuel ratio λtg is corrected to a fuel lean side by the lean accumulated amount λIL.

[0059] In a case that the last determination is the lean condition, it is assumed that the exhaust gas condition has been changed from the lean to the rich condition. The routine proceeds to S310 and 311 to set a lean skip amount λSKL and set the target air-fuel ratio λtg. In S310, the lean skip amount λIL is obtained by looking up a map defined by an estimated storage amount of the catalyst 14. The estimated storage amount of the catalyst 14 is estimated based on the signal VOX2 from the second gas sensor 27. The map is defined so that the lean skip amount λSKL is increased as the storage amount of rich components in the catalyst 14 is increased. In S311, the target air-fuel ratio λtg is corrected to a fuel lean side by the sum of the lean accumulated amount λIL and the lean skip amount λSKL.

[0060] In S312, the determination of S301 is stored in LR(i-1) as a last determination for next cycle of the routine.

[0061] The ECU 30 further executes a storage amount control as shown in FIGS. 6 and 7. FIG. 6 is a flowchart showing storage amount control. FIG. 7 is a timing diagram showing conditions of the engine 1 under the control of the ECU 30. A routine shown in FIG. 6 is executed every predetermined interval.

[0062] Referring to FIG. 6, in S401, it is determined that whether or not a timer EXTL is equal to 0. The timer EXTL represents a period of time for fixing the target air-fuel ratio in a leaner fixed value. In S402, it is determined that whether or not the signal VOX2 is greater than a threshold value, e.g., 0.55 (V). The threshold value 0.55 (V) is a threshold value between a rich condition (fuel rich condition) and a lean condition (fuel lean condition) of the exhaust gas at a location downstream the catalyst 14. Therefore, in S402, the condition of the exhaust gas downstream the catalyst 14 is estimated by the signal VOX2 indicative of the condition of the exhaust gas upstream the catalyst 14. If the signal VOX2 is higher than 0.55 (V), the exhaust gas is the rich condition.

[0063] If the signal VOX2 is gradually increased and reaches 0.55(V) at t10 as shown in FIG. 7, the routine proceeds to S403 to accumulate the intake air amount GA. In S403, the present value of the intake air amount GA is added to the accumulated amount GASUM. In S404, it is determined that whether or not the accumulated amount GASUM is greater than a threshold value GASUMMIN. If the accumulated amount GASUM is not greater than the threshold value GASUMMIN, the routine proceeds to S411 to set a normal target value that is set by the routine shown in FIG. 5. The target air-fuel ratio λtg is regulated by the routine shown in FIG. 5 from t10 to t12 as shown in FIG. 7. In FIG. 7, the target air-fuel ratio λtg is skipped at t11 in which the signal VOX2 reaches to 0.6(V).

[0064] If the accumulated amount GASUM reaches to the threshold value GASUMMIN at t12 as shown in FIG. 7, the routine proceeds to S405 and S406 to initialize the accumulated amount GASUM and preset the timer EXTL. In S405, the accumulated value GASUM is initialized to 0. In S406, a predetermined value K is set in the timer EXTL. Then, in S407, it is determined whether or not the intake air amount GA is smaller than a threshold value GAMIN. If the intake air amount GA is maintained below the threshold GAMIN as shown in periods from t12 to t13, and from t14 to t16 in FIG. 7, the routine proceeds to S408 and S409 to decrement the timer EXTL and fix the target air-fuel ratio λtg. In S408, the timer EXTL is decremented by a predetermined value, e.g., 1. Therefore, the timer EXTL is decreased from t12 to t13 and from t14 to t16 in FIG. 7. In S409, the target air-fuel ratio λtg is fixed at a predetermined leaner value, e.g., 1.05. The predetermined leaner value is sufficient to increase oxygen concentration in the exhaust gas supplied to the catalyst 14. As a result, even if the catalyst 14 may consume all of the oxygen storage during the feedback control of the main and sub feedback controllers 30 a, 30 b, it is possible to recover the oxygen storage in the catalyst 14 by temporarily setting a leaner target air-fuel ratio.

[0065] If the intake air amount GA become not smaller than the threshold value GAMIN as shown in a period from t13 to t14 in FIG. 7, the routine proceeds from S407 to S411 to set the normal target air-fuel ratio.

[0066] If once the timer EXTL is preset in S406, the routine jumps from S401 to S407 until the timer EXTL become 0. Therefore, if once the timer EXTL is preset, the leaner fixed target air-fuel ratio is activated for a predetermined time corresponding to the predetermined value K as shown in a period from t12 to t16 in FIG. 7. If the timer EXTL is equal to 0 and VOX2 is not greater than 0.55(V), the routine proceeds to S410 and S411 to initialize the accumulated amount GASUM and set the normal value. Therefore, when the exhaust gas supplied to the catalyst 14 is maintained at the lean condition, the fixed leaner target air-fuel ratio is never activated.

[0067] According to the second embodiment, it is possible to prevent the catalyst 14 from shortage of the oxygen storage.

[0068] Third Embodiment

[0069] The third embodiment has the same components as the first embodiment. The ECU 30 executes the air-fuel feedback control by the main and sub feedback controllers 30 a and 30 b. FIG. 8 is a flowchart showing a fuel cut control including a fuel cut extending control. The routine in FIG. 8 is executed every predetermined time. In the third embodiment, the shortage of the oxygen storage in the catalyst 14 is recovered by reducing an amount of fuel supply by extending the fuel cut period. The fuel cut period can be extended by lowering a threshold value of the engine speed that permits to resume the fuel supply.

[0070] In S501, it is determined that whether or not the signal VOX2 is greater than a threshold value, e.g., 0.55 (V). If the signal VOX2 is greater than the threshold value, the ECU 30 accumulates the intake air amount GA as an accumulated amount GASUM in S502. If the signal VOX2 is not greater than the threshold value, the ECU 30 initialize the accumulated amount GASUM in S503.

[0071] In S504, a fuel cut engine speed FCNE is determined by looking up a map defined by the accumulated amount GASUM and the water temperature THW. The fuel cut engine speed FCNE is used to start the fuel cut period, and also used to stop the fuel cut period. That is, the fuel cut engine speed FCNE defines a resuming engine speed of the fuel supply. The fuel cut engine speed FCNE is lowered as the accumulated amount GASUM is increased. The fuel cut engine speed FCNE is lowered as the water temperature THW is increased. As shown in S502, the accumulated amount GASUM is increased only when the signal VOX2 indicates that the exhaust gas at downstream the catalyst 14 is the rich condition. Since the rich condition may cause a complete consumption of the oxygen storage in the catalyst 14, the accumulated amount GASUM represents possibilities of the shortage of the oxygen storage of the catalyst 14. Therefore, lowering the fuel cut engine speed FCNE in accordance with the accumulated amount GASUM extends a period of time of the fuel cut operation in accordance with the possibility of the shortage of the oxygen storage. As a result, it is possible to recover the oxygen storage amount in the catalyst 14.

[0072] In S505, it is determined that whether or not the accelerator pedal is released, that is, the engine 1 is running under an idling. In S506, it is determined that whether or not the engine speed NE is greater than the fuel cut engine speed FCNE. If the engine 1 is operated under the idling and the engine speed NE is greater than the fuel cut engine speed FCNE, the routine proceeds to S507 to suspend a fuel supply and execute a fuel cut operation. If the engine 1 is not operated under the idling or the engine speed NE is not greater than the fuel cut engine speed FCNE, the routine proceeds to S508 to finish the fuel cut operation and resume the fuel supply.

[0073] In FIG. 8, since the fuel cut engine speed FCNE can be lowered during the fuel cut operation, the period of time of the fuel cut can be extended in accordance with the possibility of the shortage of the oxygen storage in the catalyst 14. For example, as shown in FIG. 9, in the case in which the accelerator pedal is released from t20 to t25, the fuel cut operation is commenced from t20 in response to the engine speed higher than the fuel cut engine speed FCNE. Then, if the signal VOX2 increases and reaches to the threshold value at t21, the ECU 30 commence accumulating the accumulated amount GASUM. As the accumulated amount GASUM increases, the fuel cut engine speed FCNE is decreased unless the signal VOX2 is recovered below the threshold value. If the engine speed NE is lowered below the fuel cut engine speed FCNE from t22 to t23, the fuel cut operation is suspended. The fuel cut engine speed FCNE resumes to a normal value at t24 in response to the initialize of the accumulated amount GASUM by S503.

[0074] According to the third embodiment, it is possible to prevent the catalyst 14 disposed downstream another catalyst 13 from shortage of the oxygen storage.

[0075] Fourth Embodiment

[0076]FIG. 10 is a block diagram of the fourth embodiment of the present invention. The same reference numbers as the first embodiment are used to denote the same or corresponding components. A third gas sensor 28 is added to the first embodiment shown in FIG. 1. The third gas sensor 28 is an oxygen sensor to output a signal VOX3 indicative of an oxygen concentration in the exhaust gas downstream the catalyst 14. The ECU 30 determines the rich condition or lean condition of the exhaust gas downstream the catalyst 14 based on the signal VOX3. The ECU 30 also determines the change over timing between the rich condition and the lean condition of the exhaust gas downstream the catalyst 14 based on the signal VOX3.

[0077]FIG. 11 shows a flowchart showing an air supply control. The routine shown in FIG. 11 is executed every predetermined time.

[0078] In S601, it is determined that whether or not the signal VOX3 is greater than a threshold value, e.g., 0.7 (V). The threshold value indicates a threshold value between the rich condition and the lean condition of the exhaust gas downstream the catalyst 14. Since the signal VOX3 directly represents the condition of the exhaust gas downstream the catalyst, the threshold value is slightly higher than that of the first, second and third embodiments in which the signal VOX2 is used for estimating the condition of the exhaust gas downstream the catalyst 14. In S602, it is determined that whether or not the intake air amount GA is smaller than a threshold value GAMIN.

[0079] If the signal VOX3 indicates the rich condition, and the intake air amount GA is not enough, the routine proceeds to S603 to activate the air supply device 15. As a result, it is possible to recover the oxygen storage amount in the catalyst 14 in response to the rich condition of the exhaust gas downstream the catalyst 14. On the contrary, if the signal VOX3 indicates the lean condition, or the intake air amount GA is sufficient, the routine proceeds to S604 to deactivate the air supply device 15.

[0080]FIG. 12 is a timing diagram showing an example of operation of the fourth embodiment. In the case illustrated in FIG. 12, the air supply device is activated from t30 to t31 and from t32 to t33.

[0081] According to the fourth embodiment, it is possible to prevent the catalyst 14 from the shortage of the oxygen storage.

[0082] Fifth Embodiment

[0083] Fifth embodiment has the same components as the fourth embodiment. FIG. 13 shows a flowchart showing a leaner target air-fuel ratio setting control. The routine shown in FIG. 13 is executed every predetermined time.

[0084] In S701, it is determined that whether or not the signal VOX3 is greater than a threshold value, e.g., 0.7 (V). In S702, it is determined that whether or not the intake air amount GA is smaller than a threshold value GAMIN.

[0085] If the signal VOX3 indicates the rich condition, and the intake air amount GA is not enough, the routine proceeds to S703 to set the target air-fuel ratio λtg in a leaner value, e.g., 1.05. The leaner value is the same as in S409 in the second embodiment. As a result, it is possible to recover the oxygen storage amount in the catalyst 14 in response to the rich condition of the exhaust gas downstream the catalyst 14. On the contrary, if the signal VOX3 indicates the lean condition, or the intake air amount GA is sufficient, the routine proceeds to S704 to set the normal value that is the same as S411 in the second embodiment.

[0086]FIG. 14 is a timing diagram showing an example of operation of the fifth embodiment. In the case illustrated in FIG. 14, the target air-fuel ratio λtg is fixed at the leaner value from t41 to t42 and from t44 to t45.

[0087] According to the fifth embodiment, it is possible to prevent the catalyst 14 from the shortage of the oxygen storage.

[0088] Sixth Embodiment

[0089] Sixth embodiment has the same components as the fourth embodiment. FIG. 15 shows a flowchart showing a fuel cut control. The routine shown in FIG. 15 is executed every predetermined time.

[0090] In S801, a fuel cut engine speed FCNE is determined by looking up a map defined by the signal VOX3 and the water temperature THW. The fuel cut engine speed FCNE is decreased as the signal VOX3 is increased to a fuel rich side. The fuel cut engine speed FCNE is also decreased as the water temperature THW is increased. As a result, since the signal VOX3 indicates a possibility of the shortage of the oxygen storage amount in the catalyst 14, the fuel cut engine speed FCNE is lowered as the possibility of the shortage is increased. In S802, it is determined that whether or not the engine is operated under the idling. In S803, it is determined that whether or not the engine speed NE is higher than the fuel cut engine speed FCNE.

[0091] If the fuel cut is permitted by S802 and S803, the routine proceeds to S804 to suspend fuel supply and execute the fuel cut operation. If the fuel cut is prohibited by S802 or S803, the routine proceeds to S805 to stop the fuel cut operation and resume the fuel supply.

[0092]FIG. 16 is a timing diagram showing an example of operation of the fifth embodiment. In the case illustrated in FIG. 16, the fuel cut operation is carried out from t50 to t51. The fuel cut operation is carried out in a extended manner by lowering the fuel cut engine speed FCNE in accordance with the signal VOX3. The longer period of time of the fuel cut operation increases the fuel lean components in the exhaust gas, and recovers the oxygen storage amount in the catalyst 14.

[0093] According to the sixth embodiment, it is possible to prevent the catalyst 14 from the shortage of the oxygen storage.

[0094] Alternatively, a partial cylinder operation of the engine 1 may be used to recover the oxygen storage in the catalyst 14 in addition of the above described embodiment or instead. of the air supply operation, the leaner target operation or the fuel cut extending operation in the above described embodiment. In addition, two or more recovering method may be used simultaneously.

[0095] Alternatively, three or more catalysts may be disposed on the exhaust passage. In this case, the present invention may be applied to not only the most downstream catalyst but also the catalyst located on downstream to another catalyst.

[0096] In addition, the first, second and third gas sensors may be provided by either the air-fuel ratio sensor or the oxygen sensor.

[0097] Although the present invention has been described in connection with the preferred embodiments thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Such changes and modifications are to be understood as being included within the scope of the present invention as defined in the appended claims. 

What is claimed is:
 1. An air-fuel ratio control apparatus for an engine comprising: a first catalyst disposed in an exhaust passage of the engine; a second catalyst disposed in the exhaust passage downstream the first catalyst; an air-fuel ratio sensor disposed in the exhaust passage upstream the first catalyst; second catalyst condition detecting means for detecting a condition of the second catalyst; feedback control means for controlling an air-fuel ratio to maintain a detected air-fuel ratio by the air-fuel ratio sensor in a target air-fuel ratio by using a feedback control method, the target air fuel ratio being set in accordance with an output of the second catalyst condition detecting means; and oxygen storage recovery means for recovering an oxygen storage in the second catalyst when the second catalyst condition detecting means outputs a predetermined level.
 2. The air-fuel ratio control apparatus for an engine according to claim 1, wherein the second catalyst condition detecting means detects an air-fuel ratio of the exhaust gas in the exhaust passage downstream the second catalyst, or a rich condition and a lean condition of the exhaust gas in the exhaust passage downstream the second catalyst.
 3. The air-fuel ratio control apparatus for an engine according to claim 1, wherein the oxygen storage recovery means increases an oxygen concentration in the exhaust gas supplied to the second catalyst when the second catalyst condition detecting means outputs a signal that indicates a rich condition of the exhaust gas.
 4. The air-fuel ratio control apparatus for an engine according to claim 3, further comprising an air supply device that supplies the air to the second catalyst, wherein the oxygen storage recovery means increases an oxygen concentration in the exhaust gas supplied to the second catalyst by activating the air supply device.
 5. The air-fuel ratio control apparatus for an engine according to claim 3, wherein the oxygen storage recovery means increases an oxygen concentration in the exhaust gas supplied to the second catalyst by at least reducing a fuel amount supplied to the engine.
 6. The air-fuel ratio control apparatus for an engine according to claim 5, wherein the fuel amount is reduced by adjusting the air-fuel ratio of the exhaust gas supplied to the second catalyst in a lean condition.
 7. The air-fuel ratio control apparatus for an engine according to claim 6, wherein the air-fuel ratio is adjusted in a lean condition by setting the target air-fuel ratio in the feedback control means in a lean value.
 8. The air-fuel ratio control apparatus for an engine according to claim 5, further comprising means for suspending a fuel supply for all or partial cylinder and resuming the fuel supply when an engine speed reaches to a resuming engine speed, wherein the fuel amount is reduced by adjusting the resuming engine speed lower than a normal value.
 9. The air-fuel ratio control apparatus for an engine according to claim 1, wherein the feedback control means comprises: main feedback means for maintaining the detected air-fuel ratio in the target air-fuel ratio by using the feedback control method; and sub feedback means for setting the target air fuel ratio in accordance with the output of the second catalyst condition detecting means. 